How could the universe arise from vacuum fluctuations?

[girl914]

I apologize if this is a "stupid" question, but I have been trying to find information on this for weeks and it has been to no avail thus far.

I know that there are two theories in regards to what came "before" the singularity; the cyclic universe and the "universe came from a vacuum fluctuation" theory. I understand the former relatively well, but I just do not understand the latter, and was wondering if someone could offer a hypothetical explanation.

I understand that vacuums can be responsible for the temporary creation of particles. What I do not understand is how those particles could expand and become our current universe. Is the answer just "it has something to do with quantum gravity, which we do not understand," or is there a hypothetical situation in which the above could occur? If my conclusions are erroneous, please explain.

Thank you so much.

 

[audioloop]

about cyclic universeshttp://arxiv.org/pdf/1204.4658v1.pdf
Audrey Mithani, Alexander Vilenkin

...The first two of these scenarios are geodesically incomplete to the past, and thus
cannot describe a universe without a beginning. The third, although it
is stable with respect to classical perturbations, can collapse quantum
mechanically, and therefore cannot have an eternal past...http://arxiv.org/pdf/1204.5385v1.pdf
Leonard Susskind

...Combing the Mithani-Vilenkin's observations with the ones in this note, we may
conclude that there is a beginning...

 

[Chronos]

It is possible the universe emerged from some prior state that was fundamentally different from any state known to exist today, but, it is unclear if that hypothesis is testable. The classic cyclical universe model has thermodynamic issues.

 

[Mark M]

Well, I disagree a bit with your categorization of pre-big bang models. There are an enormous number of different theories, the 'from nothing' part is usually associated with different models. For example, chaotic inflation holds that inflation began out of a vacuum, producing a universe. Sean Carroll's model also involves universe production out of a vacuum, but in his model, an eternal, cold, de Sitter space perpetuates the production of the universes. There are also a plethora of cyclic models, not just one or two.

So, the vacuum fluctuation is a part of many cosmologies. It's based off of a widely accepted theory called inflation - in inflation, the early universe is filled with a scalar field called the inflaton field. The field's potential energy curve would be such that it had to overcome a large potential energy 'hill' to reach the lowest energy state. This is called a false vacuum. One example is supercooled water, water that is cooled below it's freezing point. It wants to get to the energetically favorable frozen state, but doesn't have enough energy to undergo the freezing process. It is incredibly unstable, disturbing supercooled water will cause it to freeze.

This desirable energy state, a solid in the case of supercooled water, is called a true vacuum. The inflaton field wants to reach the true vacuum, but it doesn't have sufficient energy to get out of it's false vacuum. So, it was stuck.

It turns out that a scalar field in a false vacuum produces an enormous negative pressure. This functions as an extremely powerful expansion, expanding the universe by unthinkable amounts. After a short period of time, quantum tunneling allows the inflaton field to reach it's true vacuum, and inflation ends. Also, the inflaton field then decays away into a hot bath of radiation, reheating the universe to a sufficient temperature for big bang nucleosynthesis.

So, there are a few models of inflation that allow it to begin from a vacuum. There is chaotic inflation, for one. In this model, the inflaton field is in a normal state, and the universe is cold and empty. Quantum tunneling allows the inflaton field to get into it's false vacuum - even though classically this is impossible, it can occur in quantum mechanics. From there, inflation takes over.

The other idea that has gained popularity is the Fahri-Guth mechanism. The idea is that an inflationary false vacuum can form in empty space. In their paper (which is unfortunately not available on the internet), they showed that such a state was unstable, and should collapse - and actually could tunnel out of it's space-time to prevent a naked singularity from forming. This would become a new space-time, and inflate as according to normal inflation. This 'baby universe' would be connected to the original universe through a wormhole. Since these are unstable, it will collapse, and the mouths in each universe will become black holes in each universe. They will then be totally separated.

 

[marcus]

I apologize if this is a "stupid" question, but I have been trying to find information on this for weeks and it has been to no avail thus far.

I know that there are two theories in regards to what came "before" the singularity...

You are going to get conflicting information. I hope you don't mind this. In science the really interesting questions are often surrounded by people confidently asserting conflicting ideas.

If you have a few moments to peek into the collective mind of the world's experts (chaotic as it is) you could glance at the list of topics at this big triennial conference that just happened in Stockholm. Over a thousand participants this time!
http://www.icra.it/mg/mg13/
The list of parallel sessions (don't bother only in case of curiosity):
http://www.icra.it/mg/mg13/parallel_sessions.htm

Nonsingular cosmology made a strong showing this time. Typically that means a big bounce cosmology where a contraction rebounds, starting the expansion that we see. No attempt to explain where the contracting universe "came from". That's a problem for another day. The problem of why existence exists is for philosophers and the general public to think about. these people just want to understand the mechanism underlying the start of the expanding universe we see.

There was not much about "quantum fluctuations" or about the ideas of people like Hawking, Vilenkin that go back to the 1980s and 1990s. Fashions change. I really think there is no dominant model at present. People will talk to you in tones of absolute certainty but a lot of it is still just hypothetical and speculative. The picture of the human mind grappling with the start of expansion is neither pretty or simple.

I have faith that beauty and clarity will emerge out of the current chaos. I trust us to manage this.

Here is my personal favorite cosmology session of the Marcel Grossmann meeting:
http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find

I'm particularly excited by the work that Francesca Vidotto and Andrea Dapor presented at that session!
And Francesca is a PF member who has recently been contributing to discussion at the "Beyond Standard" forum! She is too busy to be around here regularly but comes in once and a while. Here is one of francesca's posts https://www.physicsforums.com/showthread.php?p=3998710#post3998710
She has 8 or 10 more in that thread as I recall.
Andrea and two other people just released a preprint of some research where they showed how you could have a bounce cosmology in which the inflation right after the rebound was caused by the Higgs field. Nice result!
I can't speak with any kind of assurance but I really like what all the people in that particular conference session are doing. They are a young bunch. Mostly postdocs.

Can't resist pasting in the abstract of one of the two presentations that Andrea gave during that session:
http://ntsrvg9-5.icra.it/mg13/FMPro...s&talk_accept=yes&-max=50&-recid=42162&-find=

Speaker
Dapor, Andrea
Co-authors
Michal Artymowski, Tomasz Pawlowski
Talk Title
Loop Quantum Cosmology for nonminimally coupled Scalar Field
Abstract
We conduct a LQC-quantization of the FRW cosmological model with nonminimally coupled scalar field. (This model is interesting from the classical point of view because it allows heavy fields (such as the Higgs) to produce inflation.) Making use of a canonical transformation (between Jordan variables and Einstein variables), we recast the theory in a minimally coupled one, for which standard LQC techniques can be applied to find the physical Hilbert space and the dynamics. Though the analysis of the genuine quantum system can be performed, we focus on the semiclassical sector - obtaining a "classical" effective Hamiltonian. At this level, we can transform back to the Jordan frame, and study the dynamics. It turns out that the initial singularity is replaced by a "mexican hat"-shaped bounce, joining the contracting and expanding branches.
==============

If you click on any of the talk titles in the program you get an abstract summary of the talk so you can check to see, for example, what Francesca was talking about. She also gave a talk at another sessions like QG1 A.
http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find

 

[marcus]

Here is another session of the Stockholm conference that dealt with nonsingular cosmologies. Mostly bounce I gather.
http://ntsrvg9-5.icra.it/mg13/FMPro...tField=order2&-SortOrder=ascend&-Max=50&-Find

I had forgotten, I see Andrea gave a talk at this session too---same stuff Loop bounce with Higgs-driven inflation.

Not much of the older ideas of "quantum fluctuation" and "eternal inflation" were in evidence anywhere as far as I can tell.

There is another triennnial series of big international conferences that deal with cosmology, the big bang, the geometry of the universe and such like topics. As it happens that one will be held NEXT YEAR in Warsaw starting July 8. This is the socalled "GR" conference.
Next year will be the 20th in the series so it will be called "GR 20".

These are the two main big international conferences where you can see what the current state of the field is. Who chairs the sessions, what the lineup, what are the hot topics etc.
So we will get a chance next year to check and see if our impressions hold up of how things are going.

 

[phinds]

In science the really interesting questions are often surrounded by people confidently asserting conflicting ideas.

How EXTREMELY well said, Marcus.

 

[twofish-quant]

Also confidentally asserting conflicting ideas is how science proceeds. A lot of science involves "boxing matches" between rival ideas, and when you are in a "boxing match" you are going to push your idea as hard as you can, knowing that other people are going to push their ideas as hard as they can.

One thing that's interesting is that physicists often communicate in a peculiar way that confuses people that aren't used to it. A lot of communication is to not only communicate facts but also emotional state. You not only are trying to communicate an idea but what you think about that idea. The thing about "boxing matches" is that you try to strip out the "emotional" parts of communications. So when a physicist talks about it idea, it often sounds like they are 100% sure of it, even if they aren't, because having two sides communicate "as if" they are 100% sure of something is how you end up with intellectual boxing.

 

[twofish-quant]

One other cool thing is that it will ultimately get settled with data. We have a lot of detailed maps of the cosmic microwave background, and we have enough data to figure out what happened when the universe inflated, and there is a good chance that there is something in the data that will tell us about the pre-inflationary state of the universe.

To put it poetically, the big bang was loud and produces a lot of sound waves which we can observe because the sound was frozen in various places in the universe. We can look at the sound of the big bang and then figure out what happened.

The hard part of coming up cosmological ideas is not to come up with ideas. That's easy. The hard part is go show that they are wrong. If you can't show that an idea is wrong, then it's sort of pointless to spend lots of time coming up with the idea. The cool thing is that we are getting flooded with data now.

 

[audioloop]

as for penrose cyclic model

http://arxiv.org/pdf/1012.1268v1.pdf

...Thus, we conclude that there is no evidence for the CCC model in the current WMAP data...

http://arxiv.org/pdf/1012.1305v3.pdf

...However, we find that this variation is entirely expected in a sky which contains the usual CMB anisotropies. In other words, properly simulated Gaussian CMB data contain just the sorts of variations claimed. Gurzadyan & Penrose have not found evidence for pre-Big Bang phenomena, but have simply re-discovered that the CMB contains structure...http://arxiv.org/pdf/1012.1656v1.pdf

...By comparing with Monte Carlo simulations of the CMB sky, we find that the low variance circles of Gurzadyan & Penrose (2010) are not anomalous...

 

[audioloop]

as for cyclic models at whole

http://arxiv.org/pdf/gr-qc/0701070v1.pdf

...We show that, contrary to expectation, it is unlikely that such models can offer a physically viable solution to the flatness problem...

...This behavior is well known, and is often used to model non-singular oscillating cosmologies. However, we find that the continual transfer of energy required to create cycles of indefinite size and duration results in subsequent increases in the non-zero minimum of expansion. Thus, by sourcing energy from this negative energy scalar field it becomes more negative, and increases in the maximum of expansion are accompanied by increases in the minimum of expansion. This behavior does not appear to be consistent with a physically viable cosmology...as for loop models
http://arxiv.org/pdf/1204.5604.pdf...However this modified Friedmann equation could have been obtained in an inconsistent way, what means that the obtained results from this equation, in particular singularity avoidance, would be incorrect. In fact, we will show that instead of a nonsingular bounce, the big rip singularity would be replaced, in loop quantum cosmology, by other kind of singularity.

...And thus, the current statement that, in loop quantum cosmology, the big rip singularity is replaced by a non-singular bounce would be incorrect...
.

 

[marcus]

as for cyclic models at whole
http://arxiv.org/pdf/gr-qc/0701070v1.pdf

as for loop models
http://arxiv.org/pdf/1204.5604.pdf
...

Both papers deal with restricted scenarios based on very special assumptions. They are not relevant to nonsingular cosmology (where the "big bang" is replaced by a bounce) in general.

Not sure what you mean by "cyclic models at whole". The word "cyclic" is not used to describe Loop models in general because often the models studied have only one bounce---they thus have no periodic cycle. Some versions do cycle, but it is not typical. Moreover the word "cyclic" is used by other cosmologists to describe a lot of different sorts of models. Steinhardt and Turok have called their "braneworld" scenarios cyclic.

In any case the paper you cite (by Barrows and Clifton) does NOT apply categorically to everything called "cyclic". It is based on restrictive specialized assumptions referring to some earlier research. So it is somewhat misleading to suggest that it applies to some ill-defined category "as a whole".

Your "as for loop" paper by de Haro has nothing to do with ordinary Loop cosmology models. It is about "phantom energy/big rip" scenarios where you do not have the usual cosmological constant.

However thanks for your interest and for looking up the research papers.

 

[Naty1]

I understand that vacuums can be responsible for the temporary creation of particles. What I do not understand is how those particles could expand and become our current universe.

If you understand the first part, 'temporary particles', please explain to me! [LOL]

For the record, nobody really knows that! We believe Heisenberg uncertainty and quantum jitters at Planck scales [quantum foam] sets some limits at small scales, but why is that?? And why do those models break down at the Big Bang and at the singularities of black holes?? Something is wrong but a solution, a more fundamental explanation, awaits a theory of quantum gravity.

Consider this a corollary to MarkM's post above:

An observer can detect new particles three ways: by simply accelerating herself, by observing an accelerating/expanding spacetime, by observing an accelerating causal horizon. Are those real of just mathematical constructs?/ I am not sure.
But I do know different spacetimes [Rindler, Schwarszschild, de Sitter,etc,etc] provide different perspectives.

let's take the first: an inertial observer and an accelerating observer passing by in free space will measure different temperatures...and make different particle counts...because their horizons are different. Similarly, accelerating outside a black hole horizon enables you to observe different temperatures and particles than if you are inertial [free falling]; in the former you get fried by radiation, in the latter you pass the theoretical horizon and are completely unaware of it! Supposedly all that is 'real' .

Next consider this:

... uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity...we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”

that's from Carlo Rovelli, this discussion and his 2004 research paper:

https://www.physicsforums.com/showthread.php?t=386051

http://arxiv.org/abs/gr-qc/0409054

So just post a question, 'What is a particle?", or better yet, see past discussions here, and youll see we have a lot of good ideas, but exactly why we have the particles we do, why they have the characteristics they do, is not really known from fundamental first principles. We know all that stuff because we measure it...not because we have a perfect underlying theory.

Finally, and briefly, the horizons associated with acceleration: The existence, properties, and significance of a cosmological/causal horizon depend on the particular cosmological model being discussed...analogous to my comment above about different spacetimes offering different perspectives.

Here is the way I first thought about particles popping out [becoming real] as a result of the presence of horizons: If you think of virtual particles as real and imaginary numbers, like some solutions to quadratic equations, Consider that the imaginary ones pass through a horizon...so they have no further causal effect..they no longer offset the 'real' number and that becomes a detectable particle. This explanation will draw lots of criticism, I'm sure, but it got me started...so maybe it will benefit you,too. I think I got the idea from the also simplistic description of Hawking radiation where a black hole horizon separates virtual particles with one disappearing behind the horizon and the other emerging as a real [detectable] particle. [This also is an intutive description according to Hawking himself when he tried to figure out what his own math might be suggesting.]

 

[marcus]

Phinds, thanks for the supportive comment!
Naty, I am told that it is not the "Higgs particle" that gives things mass, it is the Higgs FIELD, that extends throughout space.
The particle is a manifestation of the field that occurs at certain energies in certain reactions the products of which appear in certain detectors. It is a quantum of the field. I think maybe we should think more in terms of the field sometimes.
So I very much like this part of your post:
==quote Naty==
...
...Next consider this:

... uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. More in general, particle states are difficult to define in a background-independent quantum theory of gravity...we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”​

that's from Carlo Rovelli, this discussion and his 2004 research paper:

https://www.physicsforums.com/showthread.php?t=386051

http://arxiv.org/abs/gr-qc/0409054
==endquote==

 

[audioloop]

Not sure what you mean by "cyclic at whole". The "cyclic" is not to describe Loop in general

who say LOOP in the first paper ?

 

[Mark M]

sure ?http://arxiv.org/pdf/1207.3621v1.pdf

Does loop quantum cosmology replace the big rip singularity by a non-singular bounce?...Our main conclusion is that in loop quantum cosmology, contrary to the current belief, the big rip singularity is not replaced by a non-singular bounce...

What? Didn't you read the title of that paper? It states that a big rip singularity cannot be avoided in LQG. Which is perfectly fine, because very few believe the big rip is even a viable scenario.

Marcus is speaking of the big bang singularity at t=0. If you read the abstract, they note that LQG resolves the big bang singularity.

Why would you post a paper for a subject that it doesn't involve?

 

[audioloop]

They are not relevant to nonsingular cosmology (where the "big bang" is replaced by a bounce) in general.

sure ?http://arxiv.org/pdf/1207.3621v1.pdf

...that this singularity is replaced by a non-singular bounce...

 

[Mark M]

sure ?http://arxiv.org/pdf/1207.3621v1.pdf

...that this singularity is replaced by a non-singular bounce...

I corrected this above, it refers to the big rip singularity, NOT the big bang singularity. Why did you repost it?

 

[audioloop]

LQG resolves the big bang singularity.

...It is therefore a common hope that quantum effects, for instance new repulsive contributions to the gravitational force, can help to avoid singularities...

 

[skydivephil]

Both papers deal with restricted scenarios based on very special assumptions. They are not relevant to nonsingular cosmology (where the "big bang" is replaced by a bounce) in general.

Not sure what you mean by "cyclic models at whole". The word "cyclic" is not used to describe Loop models in general because often the models studied have only one bounce---they thus have no periodic cycle. Some versions do cycle, but it is not typical. Moreover the word "cyclic" is used by other cosmologists to describe a lot of different sorts of models. Steinhardt and Turok have called their "braneworld" scenarios cyclic.

In any case the paper you cite (by Barrows and Clifton) does NOT apply categorically to everything called "cyclic". It is based on restrictive specialized assumptions referring to some earlier research. So it is somewhat misleading to suggest that it applies to some ill-defined category "as a whole".

Your "as for loop" paper by de Haro has nothing to do with ordinary Loop cosmology models. It is about "phantom energy/big rip" scenarios where you do not have the usual cosmological constant.

However thanks for your interest and for looking up the research papers.

As far as I can see LQC only tells us the big bang is replaced by a big bounce. But it doesn't have anything to say about the future of our universe or what might have been the origin (if any) of the universe that we bounced from. Hence it has sometHing in common with cyclic models in that the big bang is not the ultimate beginning but it doesn't give a complete cyclic description in the same way as , for example, Ekpyrotic or CCC models do. However I did see Param Singh on a BBC documentary describe it as cyclical, so I am wondering is there something I have missed or is there more diversity within the field or...?

 

[marcus]

As far as I can see LQC only tells us the big bang is replaced by a big bounce. But it doesn't have anything to say about the future of our universe or what might have been the origin...

However I did see Param Singh on a BBC documentary describe it as cyclical, so I am wondering is there something I have missed or is there more diversity within the field or...?

Hi Skydive, there are versions that cycle. Basically just have to adjust the parameters so that expansion stops and the universe recollapses. Then there will be another bounce, and another bounce etc.

But this does not typify the behavior in general. You can also adjust the parameters so that expansion continues indefinitely, and the whole history has only one bounce.

Something that Ashtekar has been stressing in recent papers is that you can adjust the parameters of the LQC model so that its results are consistent with WMAP7 data. Then you can make predictions about what will be seen in further observation, say by Planck. That way you have a test.

This is what you want any cosmology model to do. You want to be able to fit the model quantitatively to existing data and then make quantitative predictions about future data, so you can check.

The word "cyclic" is not used to describe Loop models in general because often the models studied have only one bounce---they thus have no periodic cycle. Some versions do cycle, but it is not typical.

Back in 2006 when the Penn State people were running lots of simulations with the new LQC dynamics they would OFTEN show the repeated bounce version results. When they published they would have plots of a single bounce and also a cyclic bounce.

That kind of behavior has been studied a lot. But it is not true of LQC models in general. I don't know what Param Singh might have said. He may have spoken loosely referring to some or many of the computer runs he was doing at the time. A lot of the plots they published around 2006 and 2007 did cycle, rather beautifully in fact.

The new LQC dynamics was first developed in 2006 and when they started simulating universes in the computer with it they did not care so much about realism. They did not introduce inflation at first. They didn't do late time acceleration. They were just exploring to see basic things like "do you always get a bounce? or does it go haywire in some cases?" and "what density does it have to reach before it bounces?"

More recently they have been introducing more realistic features and getting more detailed agreement with observation. So if you interviewed Singh now he probably would not stress the "cyclic" results. At least I hope he wouldn't

I think you have the basic idea right: it's not about where the universe ultimately came from. It is more focussed on understanding our own particular bounce that we think happened 13.7 billion years ago and which produced the fluctuations in the ancient light which we see today, and from which ensued the variations in matter density that we see today. Consequently it's about making predictions about finer detail in the CMB that one may look for in the next round of observation, or the one after that.

It's not about answering questions of Existence and Ultimate Destiny. Clearly you can't even begin to do that until you have a reliable testable model of what immediately preceded the start of expansion. Besides, maybe we'll never understand E and U.D. It would be great just to have a better understanding of the birth of this particular expansion that we see around us and are part of. this is running on, I should stop

 

[skydivephil]

Thanks Marcus, as always a fascinating reply. I didnt know there were any loop models that had anything to say about our unvierse recollapsing. I know there are non loop cyclic models that generate a mechanism for recollapse such as a spring like force in a hgiher dimension in Ekpyoritc model, re scaling in CCC etc. But in those loop scenarios that do have cycles, what is that makes the acceleration of the universe we currently observe slow down and turn around?

Also Planck's cosmological results are due in about 6 months, do we have any clean set of predictions from LQC that don't rely on discovering the B mode and are different to the 7 year WMAP results? I have to say my main fear for PLanck is that it will just be WMAP to more decimal places. Perhpas we need a dedicated B mode mission?

 

[Naty1]

I want to go back to the Girl914 question:

What I do not understand is how those particles could expand and become our current universe

As I think Marcus posted, it is quantum FIELDS, extended versions of particles, that expand in space with expanding space and can become 'particles', which we describe as quanta [localized versons] of fields...what we detect. There are a variety of ideas about how this might happen.

For an overview:

Gravitational fluctuations in accelerated expansion is what gives rise to the primordial perturbations. The idea is usually pitched in terms of particle/anti-particle pairs that pop out of the vacuum, only to get pulled apart by the rapidly inflating background. Eventually they are pulled outside the horizon, never to meet again. Of course, this description has the same conceptual shortcomings as the description of the Hawking effect {for Black Holes} in terms of particle/anti-particle pairs -- it isn't quite right.

and

Quantum fluctuations in the inflationary vacuum become quanta [particles]
at super horizon scales...it seems that expansion of geometry itself, especially inflation, can produce matter...The evolution of quantum fluctuations is from their birth [at Planck Scale] in the inflationary vacuum and their subsequent journey out to superhorizon scales where they become real life perturbations...[particles at detection]

So we have quantum field perturbations...what do they mean:

[These quotes may be from MTW..I did not keep the reference source]

... there is no preferred vacuum state, and the interpretation of the field states in term of particles appears to be difficult...Such arbitrariness and ambiguity of the particle concept have led some theoreticians like Davies to affirm that “particles do not exist” , a view shared by several relativists. ... other theoreticians ... hold that QFT is fundamentally a formalism for describing processes involving particles, such as scattering or decays...

This view similar to that I posted aready from Rovelli: there are some fundamental differences among theorists about what particles are...they are not easy to define.

Here is a closely related view:

... there are two distinct notions of particles in QFT. Local particle states correspond to the real objects observed by finite size detectors. ... On the other hand, global particle states...can be defined only under certain conditions. Global particle states are simpler to define and they approximate well the local particle states detected by local measurements. Therefore the global particle states, when they are available, give a good approximate description of the physics of the “real” particles recorded by local detectors...In the paper we illustrate the difference between these two classes of states, and discuss their relation. The precise sense in which global states approximate local particle states is subtle...

Here is a quote I liked from Wikipedia. Keep in mind the quantum fluctuations that are grown with accelerated cosmological expansion can be thought of as a mix of 'real and virtual particles' ...which are also field amplitudes...[analogous to the real and imagionary numbers I mentioned in my first post.]

There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. ... In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment.

That last sentence AGAIN alludes to the difficulty of defining and modeling a 'particle'. What you 'see' depends on your model.

Tom Stoer of these forums has posted:

Particles appear in rare situations, namely when they are registered.

meaning, otherwise and normally, they are fields! A related way to say this is that the concept of a "particle" is not invariant. Two observers will in general not agree on the number of particles they observe. [Also stated but differently in my first post]

more in this discussion:
What is a particle
https://www.physicsforums.com/showthread.php?t=386051

 

[marcus]

... But in those loop scenarios that do have cycles, what is that makes the acceleration of the universe we currently observe slow down and turn around?

Just simple gravity. You only get the model to behave that way when you set the mass density unrealistically high.
You may remember before 1998 when people assumed Lambda=0 the cosmology textbooks always talked about 3 distinct scenarios closed/flat/open. The "closed" scenario had the mass density above some critical value making the U both spatially closed AND destined to eventually crunch.

Now, with Lambda>0 we can have a model that is spatially closed BUT expands indefinitely.

In 2006 when the Penn State people started doing computer simulations to try out their new dynamics they kept it simple. So in effect they were back in pre-1998 cosmology. And one of the behaviors they wanted to reproduce was the "closed" case where you naturally get recollapse.

That's fine, it was a conscious choice to keep it simple and not try to do everything at once. It was also a chance to test the code and learn about behavior right around the bounce. So they ran these periodically bouncing cyclic cases. And they also ran a socalled "flat" or k=0 case where you just get one bounce and expansion if forever but just barely.

I forget when they started to put in inflation, probably around 2009 or 2010.

And Lambda only has a late time effect, in the early universe its effect is vanishingly small because radiation and matter dominated. So you include it in the theory but can neglect it if you are doing numerical simulation of behavior right around the bounce.

Also Planck's cosmological results are due in about 6 months, do we have any clean set of predictions from LQC that don't rely on discovering the B mode and are different to the 7 year WMAP results? I have to say my main fear for Planck is that it will just be WMAP to more decimal places. Perhpas we need a dedicated B mode mission?

A number of people have said we need a dedicated B mode mission. I think NASA had a request for proposals out sometime before 2008. there was talk of a "CMB-pol" mission focusing on polarization of the CMB map. You may know more about this. There was a paper by Wen Zhao and two people at Cambridge that made the argument "look we can already constrain the models with the data we have, but we could constrain them so much b better if we had an detailed map of the polarization!" But then there were budget problems and a lot of cuts.

I think what you are talking about is still on the agenda, just put on the back burner because of funding limitations. But I'm out of touch, and just infer stuff from what I read, you may know better. "CMB-pol" or sometimes referred to as "B-pol" seems like an obvious mission as soon as the funding situation improves. So I don't worry about it.

(I might worry about superrich moneypower ending democracy in America, rather than the delay of sensible science missions. )

What you say about Planck mission is interesting. I simply do not know what and how much new science to expect from Planck. I am personally very optimistic. When you say "more decimal places" that is after all what it is about. WMAP already constrains Loop cosmology.
I feel sure Planck will further narrow down the acceptable range of parameters.

You've probably seen this list of some 60 papers mostly of a Loop phenomenology character but I'll get the link in case others who haven't seen it might be reading. All these papers are recent---appeared 2009 or later. Many of these people are saying what to look for in Planck mission data. It's not, as you say, CLEAN. But that's life. A lot of people are busy on it and we'll just have to see when the dust settles. Sometimes this link is slow and I just wait. Sometimes it doesn't work but does when I come back to it later. I just tried it and it worked:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

(desy spires is better in some respects than the new system inspire)

 

[marcus]

Skydive, you follow CosmoPheno scene regularly, stay up to date pretty much. So you might like to look at this set of slides by Joel Primack of UC-SantaCruz.
It is visually kind of garish and very eclectic but he's trying to give an up-to-the-minute idea of where LCDM cosmology, confronted by observations, is.
https://dl.dropbox.com/u/5495083/LCDM-Triumphs%26Tribulations-LaPietra.pdf

His talk's title is "LCDM: Triumphs and Tribulations". You can already hear the attentiongetting sensationalism in the title. But I found it useful, it woke me up to some things.
He gave this talk just 3 days ago on 23 July.

The venue was a Templeton-funded workshop, so bound to be a bit off-center in certain philosophical directions (as you know well) as a whole. This does not mean that Primack's talk itself wouldn't be squarely mainstream. Temple-minions are smart organizers---they keep one foot in the mainstream and the other in the pulpit.

 

[Naty1]

Marcus:

Nonsingular cosmology made a strong showing this time. ...
There was not much about "quantum fluctuations"

In the Steinhart-Turok Model, a cyclic model which too many here seemed to dismiss when I posted about it, they attribute quantum fluctuations to the branes enclosing our universe. Ripples, quantum fluctuations in the flat branes, means that at crunch between the branes, slight differences in when the branes collide results in quantum fluctuations in our dimensions...

A number of people have said we need a dedicated B mode mission.

According to Steinhart-Turok, detection of B modes means if B-mode polarization IS detected this means some inflationary models pass a last and final test...and the cyclic model would not. Other tests can 'pass' a cyclic model, but not inflationary. we'll see

 

[marcus]

Naty, I'm glad to see that you have a soft spot in your heart for the Clashing Branes scenario of Steinhardt&Turok.

As you know I tend to follow current fashions and that's not necessarily a good thing. It's probably an advantage to have a balance in any group of people discussing science issues.
So you keep hold of ideas that you may have read 4 or 5 years ago, and which could in the long run prove valid!

I don't want to be dismissive of S&T BraneClash cosmology.
I admire and respect Steinhardt a lot. And his criticisms of Eternal Inflation and unlikely Fluctuation origins are really strong. His criticism of the Multiverse part of the String program is likewise strong. He's a good critic. But his BraneClash idea which I think goes back to around 2001, did not seem to catch on.

Also Turok is a very bright highly articulate person, who directs the world's top Physics/Cosmology think tank. I appreciate Turok as an important force in science, but that still does not make me think that BraneClash is interesting.

It just isn't hot now. Maybe someday it will get hot again. You could see that just from the Marcel Grossmann meeting earlier this month. A thousand experts gather in a big international conference dealing with standard, observational, and beyond-standard cosmology. They more or less ignore the idea of Clashing Branes. For that matter compared with past meetings they hardly even touch on String. They go in heavily for simple Bounce cosmology. I'm watching this collective expert mind, like a big flock of birds. It tells me what I think I should focus my interest and free time on.
Again, I'm not saying this is necessarily good. It's just part of how I operate. I'm glad you work differently.

 

[Naty1]

Marcus:

Naty, I'm glad to see that you have a soft spot in your heart for the Clashing Branes scenario of Steinhardt&Turok.

Not so much, but maybe a little!

What I really liked in their book, the Endless Universe, was that they provided an operational test to invalidate their own proposal,to distinguish it from inflationary models, and I have posted that previously. Hawking thought such a fine tuned granularity test might be available from Planck satellite measurements, but others here did not think so as of a year of so ago. I simply try keep an open mind. [My wife does not agree!]

Your comments in the prior post remind me a bit of what I have read about Fritz Zwicky...I forget the accounting source, but apparently he was often dismissed during his lifetime as a bit of a nut..perhaps an 'outcast' because he verbalized many of his ideas...apparently 3/4 of them were wrong [like other physicsts who had the self control NOT to discuss them]..yet he had the intuition to make some fantastic insights about neutron stars [I think he propsed that they probably existed] and suggested there HAD to be something like darkenergy/dark matter...

 

[Naty1]

Oh, WOW
This is a TOTAL digression, but I cannot help myself...Wikipedia says this about Fritz:
[he was basically an astronomer] Wherever I read the original accounting had me laughing out loud...

He hand-carried the Schmidt lens from Germany, which had been polished by the optician, Bernard Schmidt. In 1934 he and Baade coined the term "supernova" and hypothesized that they were the transition of normal stars into neutron stars, as well as the origin of cosmic rays.[5][6] It was a prescient insight that had tremendous impact in determining the size and age of the universe in subsequent decades.

In support of this hypothesis, Zwicky started hunting for supernovae, and found a total of 120 by himself (and one more, SN 1963J, in concert with Paul Wild) over a stretch of 52 years (SN 1921B through SN 1973K),[7] a record which still stands as of 2006 (the current runner-up is Jean Mueller, with 98 discoveries and 9 co-discoveries).

Gravitational lenses

In 1937, Zwicky posited that galaxy clusters could act as gravitational lenses by the previously discovered Einstein effect.[10] It was not until 1979 that this effect was confirmed by observation of the so-called "Twin Quasar" Q0957+561.[11]

[edit] Dark matter

While examining the Coma galaxy cluster in 1933, Zwicky was the first to use the virial theorem to infer the existence of unseen matter, which he referred to as dunkle Materie 'dark matter'.[12]...

Main article: Tired light

When Edwin Hubble discovered a linear relationship between the distance to a galaxy and its redshift expressed as a velocity,[14] Zwicky immediately speculated that the effect was due not to motions of the galaxy, but to an unknown phenomenon that caused photons to lose energy as they traveled through space.

The Wiki article misses the humor and enthusiasm displayed by Zwicky..
note the terms 'hand carried', 'posited' and 'infered'
THAT's this guy..he could not have cared less about what others thought...which drove his detractors absolutely NUTS...and the violent opposition of many of his peers who could do superior mathematical analyses but lacked his intuitive insight...thought he was a 'kook'.

 

[skydivephil]

A number of people have said we need a dedicated B mode mission. I think NASA had a request for proposals out sometime before 2008. there was talk of a "CMB-pol" mission focusing on polarization of the CMB map. You may know more about this. There was a paper by Wen Zhao and two people at Cambridge that made the argument "look we can already constrain the models with the data we have, but we could constrain them so much b better if we had an detailed map of the polarization!" But then there were budget problems and a lot of cuts.

I think what you are talking about is still on the agenda, just put on the back burner because of funding limitations. But I'm out of touch, and just infer stuff from what I read, you may know better. "CMB-pol" or sometimes referred to as "B-pol" seems like an obvious mission as soon as the funding situation improves. So I don't worry about it.

(I might worry about superrich moneypower ending democracy in America, rather than the delay of sensible science missions. )

What you say about Planck mission is interesting. I simply do not know what and how much new science to expect from Planck. I am personally very optimistic. When you say "more decimal places" that is after all what it is about. WMAP already constrains Loop cosmology.
I feel sure Planck will further narrow down the acceptable range of parameters.

You've probably seen this list of some 60 papers mostly of a Loop phenomenology character but I'll get the link in case others who haven't seen it might be reading. All these papers are recent---appeared 2009 or later. Many of these people are saying what to look for in Planck mission data. It's not, as you say, CLEAN. But that's life. A lot of people are busy on it and we'll just have to see when the dust settles. Sometimes this link is slow and I just wait. Sometimes it doesn't work but does when I come back to it later. I just tried it and it worked:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

(desy spires is better in some respects than the new system inspire)

Cant wait top check your links out this weekend. As far I am aware CMBPOL is now superceeded by the CORE, here is a link:
http://arxiv.org/abs/1102.2181

This was presented to ESA in 2011 and i don't see any updates on CMBPOL since 2010. NASA also have something in disucssion called EPIC
http://arxiv.org/abs/0906.1188

But i wouldn't hold my breath on any of these missions , probably many years befoeRE approved, once approved , WHAT DO YOU THINK? 3*10 years build time? then another few years to run the mission. I guess we need Planck to get lucky or a ground based mission to do so.